Optical sensing during low visibility conditions

This paper investigates optical camera performance in natural atmospheres with low visibility including fog, mist, haze, and precipitation. Cameras that operated in most optical wavelengths windows were tested: VIS, NIR (active GV), SWIR, MWIR, and LWIR. An eye safe lidar was also included to estimate transmission in the SWIR region. The measurements were made against contrast board targets placed at 0.5, 1, and 2 km from the sensors. The targets were heated during parts of the campaign to create a thermal contrast, which was relevant for the infrared images. For each image, the contrast, PSF, and MTF were calculated with an in-house developed analysis tool. The parameters were plotted against visibility for various weather conditions. Long wavelengths have better transmittance through haze and small particle fog. In rain, snow or dense fog, where the particles are large, the transmittance is rather independent of the wavelength as is well documented elsewhere. In haze a SWIR camera can be expected to perform better than a visual (at least as good) but since the performance of the cameras was partly masked by different updates (and quality) this does not appear quite clear in the measurement data. The active GV-camera, operating in NIR gave the best results during low daylight conditions. However, the daylight background dominated over the laser illumination during day operation. In addition to the cameras, LIDAR measurements were made to investigate how the atmospheric attenuation is estimated using a single ended measurement sensor. The LIDAR data was used to calculate the atmospheric backscatter maximum and integrated backscatter and backscatter slope. These parameters in general correlated well with the visibility readings from the weather station.